A preferred servo-valve is shown and described in detail in U.S. Pat. No. 3,549,281 which also shows in section a variable displacement van pump and the two pistons and biasing spring which adjust the position of an outer ring and the displacement of the pump.
It is an object of the present invention to provide means which can be readily added to this valve which will convert the valve to one which adjusts the displacement of the pump in response to the rate of flow of the pump output rather than pump output pressure.